AP Biology Unit 7: Natural Selection - Evolution & Adaptation | StarSpark

If you're studying for the AP Biology exam, Unit 7 is where you learn how organisms change over time. This is evolution. You need to understand natural selection, how populations change, and the evidence that supports evolution. These concepts connect everything you've learned so far. The genetic information from Unit 5, the mutations from Unit 6, and the population dynamics from Unit 8 all come together in this unit.

Topic 7.1-7.2: Introduction to Natural Selection and Phenotypic Variation

Natural selection is the mechanism that drives evolution. It's not a random process. It's a predictable consequence of how organisms interact with their environment.

Here's how it works: organisms in a population have different phenotypes. These differences are partly due to genetic variation. Some phenotypes are better suited to the environment than others. Organisms with phenotypes that help them survive and reproduce are more likely to pass those traits to the next generation. Organisms with less favorable phenotypes are less likely to survive and reproduce. Over time, the frequency of favorable phenotypes increases in the population.

Darwin called this differential survival and reproduction. Evolutionary fitness is a measure of how many offspring an organism produces relative to other organisms in the population. An organism that survives longer and produces more offspring has higher fitness. An organism that dies young without reproducing has lower fitness.

The environment constantly changes, and different environments favor different phenotypes. In a drought year, plants with deep roots might have higher fitness. In a wet year, shallow-rooted plants might have higher fitness. This is why phenotypic variation is so important. Variation gives the population options. Some individuals will have the traits needed to survive in whatever environment occurs.

Key concepts to know:

• Natural selection: A mechanism of evolution where organisms with favorable phenotypes are more likely to survive and reproduce.
• Differential survival and reproduction: Organisms with advantageous traits produce more offspring.
• Evolutionary fitness: Measured by reproductive success.
• Phenotypic variation: Differences in observable traits among individuals in a population.
• Environmental pressure: Changes in the environment select for certain phenotypes.
• Molecular variation: Variation in the number and types of molecules within cells can provide populations a greater ability to survive and reproduce in different environments.

Topic 7.3: Artificial Selection

Artificial selection is when humans deliberately breed organisms for desired traits. This is how we've developed many crops and domesticated animals. By selecting which organisms reproduce, humans change allele frequencies in populations just like natural selection does.

Artificial selection shows that populations can change rapidly when selective pressure is strong. It also demonstrates that evolution isn't just something that happened in the past. It's happening all around us in crops and domestic animals. Dog breeds, for example, are the result of artificial selection. All dogs are the same species, but selective breeding has created enormous variation in size, shape, and temperament.

Key concepts to know:

• Artificial selection: Humans deliberately selecting traits by choosing which organisms reproduce.
• Rapid change: Populations can change quickly under strong selective pressure.

Topic 7.4: Population Genetics

Evolution doesn't happen just through natural selection. Random processes also change allele frequencies in populations. These random processes are especially important in small populations.

Mutation adds new genetic variation to a population. Every organism carries some new mutations. Most are neutral or harmful, but some can eventually become common in a population.

Genetic drift is a change in allele frequencies due to chance events. In small populations, allele frequencies can change randomly, not because of natural selection. Imagine a small population with a 50-50 mix of two alleles. Just by chance, one allele might be passed on more often and become more common. The other becomes less common.

The bottleneck effect is a type of genetic drift. When a population size is drastically reduced (by disease, natural disaster, or hunting), the survivors might not have the same allele frequencies as the original population. The population "bottlenecks" to a few individuals, and genetic diversity is reduced.

The founder effect is another type of genetic drift. When a population is founded by a small number of individuals that migrate to a new location, the founder population has only some of the alleles from the original population. The founder population might have high frequencies of alleles that were rare in the original population.

Gene flow (migration) can introduce new alleles to a population. When individuals move from one population to another and breed, they introduce their alleles to the new population. Gene flow can prevent populations from diverging into separate species.

Key concepts to know:

• Mutation: Adds new genetic variation to populations.
• Genetic drift: Random change in allele frequencies, especially in small populations.
• Bottleneck effect: A type of genetic drift following a drastic population size reduction.
• Founder effect: A type of genetic drift when a population is founded by a small number of individuals.
• Gene flow (migration): Movement of individuals (and their alleles) between populations.

Topic 7.5: Hardy-Weinberg Equilibrium

The Hardy-Weinberg equation is a mathematical model that describes how allele and genotype frequencies change (or don't change) in a population. It's not a description of real populations. It's a null hypothesis.

The Hardy-Weinberg equation is: p² + 2pq + q² = 1, where p is the frequency of one allele and q is the frequency of the other allele. And p + q = 1.

The equation describes a population in Hardy-Weinberg equilibrium. Five conditions must be met for a population to be in equilibrium:

1. Large population size (no genetic drift)
2. No migration (no gene flow)
3. No new mutations
4. Random mating
5. No natural selection

None of these conditions are ever perfectly met in real populations. That's the whole point. The Hardy-Weinberg model tells you what would happen if evolution wasn't occurring. When you compare real populations to the Hardy-Weinberg prediction, you can see that evolution is occurring.

If allele frequencies are NOT changing according to the Hardy-Weinberg equation, one or more of the five conditions is being violated. Maybe the population is small (genetic drift), or maybe individuals are not mating randomly, or maybe some alleles confer better fitness than others (natural selection), or maybe there's migration happening (gene flow). The exam tests whether you can use the Hardy-Weinberg equation and recognize when a population is deviating from equilibrium.

Key concepts to know:

• Hardy-Weinberg equation: p² + 2pq + q² = 1 and p + q = 1
• Five conditions for equilibrium: Large population size, no migration, no mutations, random mating, no natural selection.
• Null hypothesis: The Hardy-Weinberg model is what you'd expect if evolution wasn't happening. Deviations show that evolution is occurring.

Topic 7.6: Evidence of Evolution

Evolution is supported by evidence from many disciplines. The exam tests whether you can recognize the different types of evidence.

Fossils provide direct evidence of evolutionary change over time. Paleontologists can date fossils using radiometric dating (like carbon-14 dating) and by looking at the age of the rocks where the fossil is found. The fossil record shows a sequence of organisms that have changed over time, with newer rocks containing fossils of organisms that look more like modern organisms, and older rocks containing fossils of more distant ancestors.

Morphological evidence comes from comparing the physical structures of different organisms. Homologous structures are structures that are similar in different species because they were inherited from a common ancestor. Your arm, a whale's flipper, a bat's wing, and a dog's leg all have the same basic bone structure, even though they're used differently. This is strong evidence of common ancestry. Vestigial structures are left-over structures that no longer serve their original function, like human appendixes or hip bones in whales. These structures suggest that organisms have evolved from ancestors that used these structures.

Biochemical and genetic evidence is some of the most convincing. DNA sequences are similar across organisms. Species that are more closely related have more similar DNA sequences. This makes sense if all organisms descended from a common ancestor. Comparisons of DNA and protein sequences can be used to determine how distantly related organisms are.

Key concepts to know:

• Fossils: Direct evidence of evolutionary change. Can be dated by radiometric methods and geological age.
• Homologous structures: Similar structures in different species inherited from a common ancestor.
• Vestigial structures: Left-over structures from evolutionary ancestors, no longer serving their original purpose.
• DNA and protein sequence similarity: More closely related species have more similar sequences.

Topic 7.7: Common Ancestry

All organisms on Earth descended from a common ancestor. This is the fundamental principle of evolution. The evidence for this is overwhelming.

Eukaryotes share structural and functional similarities that point to common ancestry. All eukaryotes have membrane-bound organelles, linear chromosomes, and genes that contain introns. These features are so specific and complex that it's nearly impossible that they would have evolved independently multiple times. It's much more likely that all eukaryotes inherited these features from a common eukaryotic ancestor.

Key concepts to know:

• Common ancestry: All organisms on Earth descended from a common ancestor.
• Shared features: Membrane-bound organelles, linear chromosomes, and introns are shared by eukaryotes and indicate common ancestry.

Topic 7.8: Continuing Evolution

Evolution is not something that happened only in the past. It's happening right now. All species continue to evolve. You can see examples all around you.

Antibiotic resistance in bacteria is a modern example of evolution in action. Bacteria that have mutations conferring resistance to antibiotics survive when antibiotics are used. Bacteria without resistance die. Over time, the population becomes resistant. This happens in months or years, not millions of years.

Similar evolution is happening with resistance to pesticides in insects, herbicides in weeds, and chemotherapy drugs in cancer cells. Pathogens are evolving and causing new diseases (emergent diseases). The fossil record also shows continuous change, not stasis (staying the same).

Genomic changes over time show that organisms are constantly accumulating new mutations. These changes accumulate and lead to evolutionary change.

Key concepts to know:

• Ongoing evolution: All species continue to evolve.
• Antibiotic resistance: Bacteria with resistance survive antibiotic treatment and become more common.
• Emerging diseases: Pathogens evolve and cause new diseases.

Topic 7.9: Phylogeny and Evolutionary Relationships

Phylogenetic trees and cladograms are diagrams that show the evolutionary relationships among organisms. They show which species are most closely related and which are more distantly related.

Phylogenetic trees show the amount of evolutionary change over time. The x-axis is time, so the longer the branch, the more change. Cladograms do not show time scale or the amount of change. They only show the pattern of evolutionary relationships (who is related to whom).

Both phylogenetic trees and cladograms are constructed using shared traits. Shared derived characters are traits that are shared by more than one species but are not found in all species. These traits indicate common ancestry. For example, hair is a shared derived character of mammals. All mammals have hair, and hair is not found in other vertebrates. Animals that share more recently derived characters are more closely related.

The outgroup is the organism that is least closely related to the rest of the organisms in the tree. It's used as a reference point to determine which traits are ancestral (old) and which are derived (new).

Molecular data (DNA and protein sequences) typically provide more accurate and reliable information than morphological traits for constructing trees. Morphological traits can be convergent (similar but not inherited from a common ancestor).

Key concepts to know:

• Phylogenetic trees: Show evolutionary relationships and the amount of change over time.
• Cladograms: Show evolutionary relationships but not time scale or amount of change.
• Shared derived characters: Traits shared by multiple species that indicate common ancestry.
• Outgroup: The least closely related organism, used as a reference point.
• Molecular data: DNA and protein sequences are usually more reliable for constructing trees.
• Hypotheses: Phylogenetic trees and cladograms represent hypotheses that are constantly being revised based on new evidence.

Topic 7.10: Speciation

Speciation is the process by which new species arise. A species is defined (using the biological species concept) as a group of organisms that can breed together and produce viable, fertile offspring.

Reproductive isolation is the key to speciation. Once populations become reproductively isolated, they can no longer exchange genes. They're separate species.

Allopatric speciation occurs when populations are geographically isolated. Physical barriers (mountains, rivers, distance) prevent gene flow. Over time, the populations accumulate different mutations and experience different selective pressures. They become so genetically different that they can no longer breed together, even if the geographic barrier is removed. They're now separate species.

Sympatric speciation occurs when new species arise within the same geographic area. This is harder to understand, but it happens. Reproductive isolation can occur without geographic barriers. For example, polyploidy in plants is a form of sympatric speciation. A plant with a chromosome number that's a multiple of the original number might not be able to breed with plants that have the original chromosome number.

Reproductive isolation can be maintained by pre-zygotic barriers (things that prevent mating or fertilization) or post-zygotic barriers (things that prevent the hybrid offspring from being viable or fertile).

The rate of speciation and evolution varies. Punctuated equilibrium is when evolution occurs rapidly after a long period of stasis (no change). Gradualism is when evolution occurs slowly and steadily over time. The fossil record shows evidence of both patterns in different organisms.

Divergent evolution occurs when a single ancestral species splits into multiple species that become adapted to different environments. This is also called adaptive radiation. Convergent evolution occurs when unrelated species develop similar adaptations to similar environments.

Key concepts to know:

• Biological species concept: Species that can breed and produce viable, fertile offspring.
• Allopatric speciation: Geographic isolation leads to reproductive isolation.
• Sympatric speciation: New species arise without geographic isolation.
• Pre-zygotic barriers: Prevent mating or fertilization (behavioral differences, temporal isolation, mechanical incompatibility).
• Post-zygotic barriers: Reduce hybrid viability or fertility.
• Punctuated equilibrium: Rapid change followed by long stasis.
• Gradualism: Slow, steady change over time.
• Divergent evolution: One species becomes many, adapted to different niches.
• Convergent evolution: Unrelated species develop similar adaptations.

Topic 7.11: Variations in Populations

Genetic diversity within a population is crucial. It's the raw material for evolution. Populations with little genetic diversity are at risk. They have fewer options for coping with environmental change.

Genetically diverse populations are more resilient to environmental perturbations. If the environment changes, diverse populations are more likely to have individuals that can survive. An allele that helps in one environment might hurt in another. This is why variation is important. Diverse populations can cope with environmental unpredictability.

The level of variation in a population determines how quickly it can evolve. Populations with high genetic diversity can respond faster to natural selection than populations with low genetic diversity.

Key concepts to know:

• Genetic diversity: Variation in alleles within a population.
• Resilience: Diverse populations can cope with environmental change.
• Evolutionary potential: Diverse populations can evolve faster.

Topic 7.12: Origins of Life on Earth

The origin of life on Earth is still not completely understood, but there's a scientific framework for thinking about it.

Earth formed about 4.6 billion years ago. The environment was too hostile for life until about 3.9 billion years ago. The earliest fossil evidence for life dates to about 3.5 billion years ago. This gives a relatively narrow window for the origin of life.

The RNA world hypothesis proposes that RNA was the earliest genetic material. DNA, as a stable molecule, came later. RNA could do two things that made it a good candidate for the first genetic material: it could replicate itself, and it could catalyze chemical reactions (some RNAs are enzymes called ribozymes). At some point, RNA-based life gave way to DNA-based life (because DNA is more stable), and proteins became the main catalysts (because they're more flexible than RNA).

The three key assumptions of the RNA world hypothesis are: (1) genetic information was originally stored and replicated by RNA, (2) base-pairing was necessary for replication, and (3) RNA molecules, not proteins, acted as catalysts.

Note: The origin of life is a scientific hypothesis, not a dogma. There are other hypotheses, and this is an active area of research. The exam tests the RNA world hypothesis specifically.

Key concepts to know:

• Early Earth timeline: 4.6 billion years ago (formation), 3.9 billion years ago (conditions suitable for life), 3.5 billion years ago (fossil evidence).
• RNA world hypothesis: RNA was the first genetic material and could replicate and catalyze reactions.
• Three assumptions: RNA stored genetic information, base-pairing was necessary for replication, RNA acted as catalysts.

Study Tips for Unit 7

StarSpark's flashcard sets for Unit 7 are organized by topic, so you can focus on whatever mechanism of evolution is giving you trouble. Try quizzing yourself on Hardy-Weinberg problems, phylogenetic trees, and speciation scenarios. Check out StarSpark's AP Biology study tools to practice these concepts interactively.

Summary: What Actually Matters for the Exam

You've covered all the topics in Unit 7. Before you move on, test yourself with these scenario-based questions. If you can answer them confidently, you're in great shape for this section of the exam.

Explore the Full AP Biology Study Guide

Unit 7 builds on the genetic variation you learned about in Unit 5 and Unit 6. Evolution is the framework that explains all of biology. Everything in the living world makes sense once you understand that all life evolved from common ancestors.

Check out the full AP Biology study plan to see how this unit connects to the rest of the course.

Other Unit Reviews:

• AP Biology Unit 1: Chemistry of Life
• AP Biology Unit 2: Cells
• AP Biology Unit 3: Cellular Energetics
• AP Biology Unit 4: Cell Communication and Cell Cycle
• AP Biology Unit 5: Heredity
• AP Biology Unit 6: Gene Expression and Regulation
• AP Biology Unit 8: Ecology
• AP Biology Exam Prep Study Guide

For official AP Biology resources, visit apcentral.collegeboard.org.

This review is aligned with the AP Biology Course and Exam Description. AP is a registered trademark of the College Board, which was not involved in the production of this guide.